#247 breaks libtaxii on windows

[mrginglymus]

#247, which mitigates CVE-2020-27197 (defence against passing a url to parse) breaks libtaxii on windows as urlparse will extract the drive letter as the scheme from absolute paths:

Python 3.8.9 (tags/v3.8.9:a743f81, Apr  6 2021, 14:02:34) [MSC v.1928 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> from libtaxii.validation import SchemaValidator
>>> SchemaValidator(SchemaValidator.TAXII_11_SCHEMA)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\dev\taxiidoom\lib\site-packages\libtaxii\validation.py", line 148, in __init__
    schema_doc = parse(schema_file, allow_file=True)
  File "C:\dev\taxiidoom\lib\site-packages\libtaxii\common.py", line 40, in parse
    raise ValueError('external URLs are not allowed')
ValueError: external URLs are not allowed
>>> SchemaValidator.TAXII_11_SCHEMA
'C:\\dev\\taxiidoom\\lib\\site-packages\\libtaxii\\xsd\\TAXII_XMLMessageBinding_Schema_11.xsd'
>>> from urllib.parse import urlparse
>>> urlparse(SchemaValidator.TAXII_11_SCHEMA)
ParseResult(scheme='c', netloc='', path='\\dev\\taxiidoom\\lib\\site-packages\\libtaxii\\xsd\\TAXII_XMLMessageBinding_Schema_11.xsd', params='', query='', fragment='')